Prerecorded optical discs are typically mass produced by injection molding or photopolymerization using stampers that have a negative relief pattern of the desired finished prerecorded optical disc. Conventionally, optical disc stampers are produced in a multi-step process that requires forming a master from a photoresist coated blank.
Referring to FIG. 1, the steps in a conventional process for forming stampers for prerecorded optical discs are depicted. The first step 10 in the process includes forming a negative image of the desired relief pattern on a blank by exposing a layer of photoresist material such as that commonly used in the semiconductor industry.
After exposure, the second step 12 requires that the exposed photoresist material be developed to provide a relief pattern that is a negative image of the desired relief pattern. In the third step 14, a layer of metal is deposited on the developed photoresist material to produce a master having a positive image of the desired surface relief pattern of the prerecorded optical disc to be produced, i.e., the relief pattern in the master contains pits where pits are desired in the finished prerecorded optical discs. In the fourth step 16, the master is then used to form one or more optical disc stampers. The stampers are typically formed by depositing metal on the master with the resulting relief pattern in the stamper being a negative image of the relief pattern desired in the prerecorded optical disc. The fifth step 18 depicted in FIG. 1 includes producing prerecorded optical data storage discs from the stampers formed in step 16.
The conventional process depicted in FIG. 1 has a number of disadvantages. Included among the disadvantages is that the photoresist material on the blanks is typically exposed with a focused laser beam in a laser servowriter to write the data in the resist material. Because the photoresist coated blank does not include tracking and timing information, the laser servowriting process must typically be performed using air bearings and sophisticated vibration isolation equipment to ensure accurate writing of the information in the photoresist material.
The photoresist-based stamper processes described above are time-consuming and expensive. As a result, turn-around time for producing prerecorded optical discs using the conventional process is limited at least in part by the time needed to form the masters and stampers.
Adding to the disadvantage of a relatively long turn-around time to produce the stampers, the processes used to form them can also produce stampers that have errors in the relief patterns formed on them. Those errors will typically not be detected until the first optical discs formed with the stampers are tested. As a result, time and money will be spent to form stampers that must then be discarded.
Additional disadvantages of the processes described above include the need to store, expose and develop the resist-coated blanks in rooms with filtered lights because of the photosensitive nature of the resist material. A further disadvantage is that the resist-coated blanks have a shelf life and must be monitored during storage to avoid degradation of the resist coating.
Another disadvantage is that the equipment needed to expose the photoresist-coated blanks must include air bearings and sophisticated vibration isolation equipment because the tracking and timing information found on the finished optical discs is not provided on the resist-coated blanks. In many instances, the equipment required to accurately expose the resist-coated blanks and electroform the masters/stampers can cost millions of dollars.
An alternative process can also be used in which a stamper is formed without forming a master. One such process is depicted in FIG. 2. In that process, a photoresist-coated blank is exposed in the first step 20. After exposure, the photoresist-coated blank is then developed in a second step 22 to form a positive image of the relief pattern needed for the optical disc stampers. After developing, a third step 24 is performed in which the developed photoresist coated blank is metallized to form a stamper having a relief pattern that is a negative image of the desired relief pattern. Stampers produced directly from photoresist-coated blanks are sometimes referred to as a first generation stampers. The stamper formed in step 24 can then be used in step 26 to produce prerecorded optical data storage discs.
Although this alternate process can reduce the time required to produce a stamper because no master is formed, it suffers from all of the other problems described above. In addition, this process raises another problem in that damage to a first generation stamper produced directly from a photoresist-coated blank will require that a new photoresist blank be exposed, developed, and metallized to form a new stamper (because the developed photoresist-coated blank is destroyed in the metallization process).
Thus, there is a need for a time- and cost-efficient method for production of optical disc stampers.